Preparation Of Modified Sludge-based Biochar And Its Adsorption Of Cd²⁺

Sludge is a waste produced by sewage treatment.In 2020,the domestic output of sludge has exceeded 60 million tons,and the main treatment methods are still incineration and landfill.Biochar is a carbon-rich material produced by the carbonization of organic raw materials under a certain thermal combustion and limited oxygen.The sludge contains a certain amount of biological organic matter and can be used to prepare biochar.The carbonization methods of biochar usually include hydrothermal carbonization,pyrolysis,and gasification.Biochar often has high carbon content,large specific surface area,abundant surface functional groups and strong ion exchange capacity.Similar to the adsorption principle of activated carbon,biochar also has adsorption capacity and can be used to adsorb heavy metals and some organic pollutants.The pore structure and surface functional groups of biochar can be improved by surface modification,and the adsorption capacity can be increased.The sludge used in the experiment was the municipal sludge in Anning city,Yunnan Province,which was pyrolyzed for 2 hours in a N₂ atmosphere at three temperatures of 400℃,600℃,and 800℃to prepare biochar,and then the surface was modified with three concentrations of NaOH solution.Characterize the composition,microscopic morphology,pore size distribution,surface functional groups,and heavy metal content of biochar,and explore the adsorption kinetics,isothermal adsorption,and adsorption experiments of biochar for Cd²⁺in aqueous solution under different initial p H conditions.The results show that as the pyrolysis temperature increases,the H/C of biochar decreases,and the mass proportion of C in the four organic elements gradually increases;the pores of biochar are mainly mesopores,with a small amount of micropores,and the specific surface area is The pore volume increases with the increase of pyrolysis temperature;800BC pore volume and specific surface area reach 0.1413 cm³·g^-1 and 65.09 m²·g^-1.But too high pyrolysis temperature will destroy the surface functional groups of biochar.The surface functional groups of 800BC are less than400BC and 600BC.The adsorption reaction of biochar to Cd²⁺is relatively slow,the reaction rate gradually decreases with time,and the reaction equilibrium can be reached in 4h.The adsorption capacity increases first and then tends with the increase of the external Cd²⁺mass concentration.In terms of stability,the adsorption effect of 600 BC is the best,and the adsorption capacity of Cd²⁺reaches 11.56 mg·g^-1.Under the condition of initial p H 2,the adsorption capacity of 600BC is only 1.31 mg·g^-1;the reason is that the presence of H⁺weakens After the precipitation reaction and ion exchange of Cd²⁺.The amount of heavy metal leaching of biochar through sulfuric acid and nitric acid method is less than the standard limit of GB 5085.3-2007"Identification Standard for Hazardous Waste Leaching Toxicity",which meets safety regulations.After the biochar was modified by NaOH solution,the micropore volume and specific surface area increased significantly.The increase effect of 600BC was the most obvious.After modification,the pore volume and specific surface area reached 0.2242 cm³·g^-1 and 123.32 m²·g-¹,respectively.The pore volume and the hydroxyl functional groups on the surface have also increased after modification.After modification,the adsorption performance of biochar for Cd²⁺is significantly increased.The higher the concentration of NaOH,the better the effect.The adsorption capacity of 600BC after 0.1mol·L^-1 modification is 25.12 mg·g^-1,at the initial p H The adsorption capacity under the p H condition of 2 is 12.71mg·g^-1,which is much higher than before modification.